1. Field of the Invention
The invention relates to a method of coating glass and, in particular, to a method of coating glass in which at least two gaseous reactants react together to form a coating on a moving ribbon of hot glass.
2. Description of the Prior Art
It is well known that coatings with desirable properties for architectural use can be produced using gaseous reactants which decompose on the hot glass surface Thus silicon coatings, useful as solar control coatings, have been produced by pyrolysing a silane-containing gas on a hot glass surface, and there have been many proposals to produce other solar control and low emissivity (high infra red reflection) coatings from other appropriate gaseous reactants. Unfortunately, it has proved difficult in commercial practice to achieve sufficiently uniform coatings of the required thickness.
UK patent specification 1 454 377 describes a process in which a gaseous mixture comprising at least one coating reactant is directed through a nozzle at a nozzle exit Reynolds number of at least 2500 against a substrate to be coated. The coating reactant, in a carrier gas, is directed on to the substrate at an angle of 90.degree. to the substrate surface through an elongated nozzle extending across the width of a ribbon of substrate to be coated, and used coating gas is withdrawn through vacuum hoods on either side of the nozzle. The coating gas is not passed parallel to the glass surface and no special provision is made for the use of mixtures of reactants which are liable to react together before reaching the hot glass surface.
According to UK patent specification 1 507 996, a uniform coating is applied from a reactant gas by causing the gas to flow parallel to the glass surface under laminar flow conditions. Again, no specific provisions are made for the use of mixtures of reactants which are liable to react together before reaching the glass surface.
UK patent specification 1 516 032 describes a process of coating glass using a fluid medium, containing one or more coating reactants which may be in liquid or gaseous form, which is directed on to the hot glass as a stream or streams at least one of which has a velocity component in the direction of movement of the ribbon and is inclined to the face of the ribbon at an angle (or mean angle) of not more than 60.degree.. Use of the invention is said to give a coating characterised by a glass-contacting stratum of homogenous structure featuring a regular arrangement of crystals. Where two or more components are required to react together, these may be supplied as separate streams through adjacent nozzles each arranged to provide a stream of reactant at an acute angle to the glass surface so that the reactants come into contact with one another in the vicinity of the glass; or a single nozzle can be used to supply a first stream of reactant while a stream of air, serving as a second reactant, is induced to flow to the reaction zone by the momentum and inclination of the first stream. An exhaust duct may be provided downstream of the coating zone to draw gas away from the coating zone, and a hood may be provided to define, with the glass surface, a flow passage over the glass away from the area of impingement of the fluid stream(s) on the glass.
UK patent specification 1 524 326 describes a process in which a gaseous medium is caused to flow along the substrate to be coated as a substantially turbulence-free layer along a flow passage defined in part by the face of the glass; the flow passage leads to an exhaust ducting via which residual gas is drawn away from the glass. The gaseous reactants are introduced into the flow passage, as in UK patent specification 1 516 032, as streams at an acute angle to the glass.
UK patent specification GB 2 026 454B is particularly concerned with a process for forming a tin oxide coating on a hot glass surface, using a gaseous medium containing tin tetrachloride in a concentration corresponding to a partial pressure of at least 2.5.times.10.sup.-3 atm and water vapour in a concentration corresponding to a partial pressure of at least 10.times.10.sup.-3 atm. In a particularly preferred embodiment, a stream of nitrogen carrier gas containing tin tetrachloride vapour is caused to flow along the glass face being coated and a stream of air containing water vapour is delivered into that stream at a position where it is in flow along the said face. A doping agent, such as hydrogen fluoride, can be fed to the substrate face separately, or mixed the moist air. The streams of gas are preferably fed to the glass surface at an acute angle of less than 45.degree., and caused to flow along the glass as a substantially turbulence-free layer along a flow passage which is defined in part by the face of the glass and which leads to an exhaust ducting by which residual gas is drawn away from the glass.
UK patent specification GB 2 044 137A describes a nozzle for use in directing streams of gaseous reactant onto a hot glass surface to be coated. In order to avoid premature reaction of the coating gases used, for example tin tetrachloride and water vapour, the nozzle includes three adjacent jet ducts each having an exhaust aperture constituted by a rectilinear slot. The jet ducts are arranged side by side with their rectilinear slots parallel and the lateral walls defining the ducts converging towards an imaginary line common to all three ducts. In use the nozzle is arranged with the slots extending across a ribbon of hot glass to be coated with the said imaginary line substantially in the plane of the glass. Discrete laminar streams of reactant flow from the ducts and impinge, along said imaginary line, on the glass. This distance between the nozzle and the glass may be reduced slightly in practice producing a relatively intense local turbulence where the gas streams impact on the glass thereby promoting mixing. Residual gases are drawn away from the coating zone by exhaust ducts upstream and downstream of the nozzle.
UK patent specification GB 2 113 120B describes a modification of the nozzle described in GB 2 044 137A in which the face of the nozzle adjacent the glass is shaped to cause flow of gases from the nozzle preponderantly in the downstream direction. The gaseous flows which are laminar as they leave the jet ducts are deflected in the direction of movement of the glass and substantially parallel to the glass. They thus strike the glass more gently than in the process of GB 2 084 137A, and the degree of turbulence is lowered, which is said to help in reducing the lack of covering which occasionally occurs with the earlier device.
In an alternative technique, from the same inventors as patent specification GB 2 044 137A and described in European patent specification EP 0 060 221, flows of coating gas are caused to run into each other before contacting the substrate either by moving them at different speeds, or by directing them towards each other at an angle greater than 35.degree., or by a combination of the above, so as to cause an almost instantaneous mixture by virtue of the stirring effect resulting from the shock. In the embodiments described, the reactant gases are delivered through a set of parallel nozzles terminating very close to the glass surface, each comprising a central pipe for a first reactant gas and a second coaxial pipe for a second reactant gas. Baffles in the pipes impart opposed rotational movements to the first and second reactant gases so that the gaseous flows meet at the mouth of the nozzle and the flows are mixed substantially instantaneously before either gas flow contacts the gas. Each nozzle additionally comprises a third pipe coaxial with the first two pipes for removal of used reaction gases from the reaction zone.
In a further method, described in UK patent application GB 2 184 748A, a coating precursor and an oxidising gas are introduced into a mixing zone well above the glass at the upstream end of a coating chamber. Heat is supplied to the mixing zone and the coating precursor and oxidising gas are thoroughly mixed in the mixing zone while exposed to the substrate hut at a height such that coating formation commences from a substantially homogeneous vapour mixture. The mixture is then caused to flow continuously through the coating chamber in contact with the upper face of the glass. It is said to be advantageous for the roof structure to reduce in height in the downstream direction, throttling the flow of vapour along the coating chamber. In some preferred embodiments, the roof structure descends as a curve leading into a downstream roof portion above the glass. This is found to promote a smooth general downstream flow of precursor laden vapour within the coating chamber which is said to be beneficial for the uniformity of the coating formed. Advantageously, the coating chamber has a length of at least 5 meters; the use of such a long coating chamber is said to be of particular benefit in increasing the coating yield when forming relatively thick coatings on a rapidly moving substrate such as a ribbon of freshly formed float glass.
Despite all the prior proposals discussed above, the applicants are not aware of any process employing gases which is used commercially for the production of coatings more than 200 nm thick on a moving ribbon of glass. There is a need for a simple process, which avoids the need for multiple nozzles liable to blockage, capable of producing substantially uniform coatings having a thickness of more than 200 nm from a mixture of gaseous reactants on a hot ribbon of float glass without the need for the long coating chambers referred to above.